Processes for the conversion of lower molecular weight alkanes such as methane to higher molecular weight hydrocarbons which have greater value are sought. One of the proposals for the conversion of lower molecular weight alkanes is by oxidative coupling. For instance, G. E. Keller and M. M. Bhasin disclose in Journal of Catalysis, Volume 73, pages 9 to 19 (1982) that methane can be converted to, e.g., ethylene. The publication by Keller, et al., has preceded the advent of substantial patent and open literature disclosures by numerous researchers pertaining to processes for the oxidative coupling of lower alkanes and catalysts for such processes.
In order for an oxidative coupling process to be commercially attractive, the process should be capable of providing a good rate of conversion of the lower alkanes with high selectivity to the sought higher molecular weight hydrocarbons. Since conversion and selectivity can be enhanced by catalysts, catalytic processes have been the thrust of work done by researchers in oxidative coupling.
Two general types of oxidative coupling processes are the sequential, or pulsed, processes and the cofeed processes. In the cofeed processes, an oxygen-containing gas and an alkane-containing gas are simultaneously fed to a reaction zone. The sequential processes are characterized by alternately cycling an oxygen-containing gas and an alkane-containing gas to a reaction zone containing the catalyst. The sequential processes have an advantage in that the reactant and hydrocarbon products are not in contact with gas phase oxygen and this results in a minimization of the undesired and unselective homogeneous oxidation of the reactant or hydrocarbon products and in the ability to avoid potentially explosive mixtures of hydrocarbon and oxygen. Also, air can be used as the source for the oxygen-containing gas.
Group VIII metals have been proposed as components in oxidative coupling catalysts, but their potential has been severely limited. Keller, et al., supra, evaluated numerous metal components for oxidative coupling in a pulsed mode system. They concluded in FIG. 6 that iron, nickel, copper, silver and platinum have, in the pulsed system, no activity above that of bare support and that cobalt possibly has a small activity above that of bare support.
Mitchell, et al., in U.S. Pat. Nos. 4,172,810; 4,205,194 and 4,239,658 propose multicomponent catalysts containing a Group VIII noble metal having a molecular weight of 45 or greater, nickel or a Group IB noble metal having an atomic number of 47 or greater; a Group VIB metal oxide and a Group IIA metal on a support for methane coupling via a sequential process. They propose that the catalyst can further contain, inter alia, iron, cobalt or a metal of the actinide or lanthanide series. They opine that Group VIII noble metal, nickel or Group 1B noble metal would dissociatively chemisorb methane; Group VIB reducible metal oxides would be reduced by adsorbed hydrogen and thus produce water, and Group IIA metal oxides would convert the adsorbed methane to carbides. The postulated carbides were stated by the patentees to be intermediates in the formation of aromatic compounds. The catalysts are described as being supported on a refractory support such as alumina. The catalyst is disclosed as being operated in a sequential (or pulsed) mode in which the oxygen containing gases and methane containing gases are alternatively cycled to the reaction zone. The catalyst becomes coked with use and therefore requires periodic regeneration.
Garcia, et al., "Direct Catalytic Synthesis of Ethylene from Methane", React. Kinet. Catal. Lett., Vol. 28, 481 (1985), disclose the use of, e.g., platinum and cobalt containing catalysts for oxidative coupling. The authors operated in the sequential mode and noted that long induction periods are required between detecting higher hydrocarbon products.
"It is interesting to note the long inductive periods required for achieving detectable conversions. ". . . This would indicate that ehtylene (sic) production can only occur when a significant surface carbon concentration is reached." (p. 434).
The role of Co is postulated to ". . . chemisorb CH.sub.4 dissociatively and provide additional surface species," (p. 435) and play ". . . the role of oxygen donor. . ." (p. 435).
Workers have reported numerous transition metal oxides supported on silica as methane coupling catalysts in sequential processes. See, for instance, U.S. Pat. No. 4,443,644 (Sb.sub.2 O.sub.3); U.S. Pat. No. 4,443,645 (GeO.sub.2); U.S. Pat. No. 4,443,646 (Bi.sub.2 O.sub.3); U.S. Pat. No. 4,443,649 (PbO); U.S. Pat. No. 4,443,648 (In.sub.2 O.sub.3); U.S. Pat. No. 4,443,649 (Mn.sub.3 O.sub.4); U.S. Pat. No. 4,444,984 (SnO.sub.2); U.S. Pat. No. 4,489,215 (Ru oxide); U.S. Pat. No. 4,593,139 (Ru oxide); and GB No. 2156842 (Mn, Sn, In, Ge, Pb, Sb, and Bi - oxides). In a summary (J. S. Sofranko, et al., J. Catal., 103, 302 (1987)), manganese-silica catalysts were reported to give the best C.sub.2 (ethylene and ethane) yields. Alkali metal and alkaline earth metal doping, especially sodium, of the manganese-silica catalysts (C. A. Jones, et al., J. Catal., 103, 311 (1987)) is reported to enhance the methane coupling ability. A 15% Mn-5% Na.sub.4 P.sub.2 O.sub.7 -silica catalyst reportedly gave 17% yields of C.sub.2 and higher hydrocarbons for 2 minute runs at 850.degree. C. in a sequential process. The beneficial effect of sodium addition is postulated by the authors to be due to increased surface basicity, reduction of surface area and a specific manganese-sodium interaction.
Numerous materials have been reported as dopants, supports, promoters and stabilizers for the manganese based methane coupling catalyst. See, for instance, U.S. Pat. No. 4,495,374; U.S. Pat. No. 4,499,322; U.S. Pat. No. 4,544,784; U.S. Pat. No. 4,544,785; U.S. Pat. No. 4,544,786; U.S. Pat. No. 4,547,608; U.S. Pat. No. 4,547,611; U.S. Pat. No. 4,613,718; U.S. Pat. No. 4,629,718; U.S. Pat. No. 4,650,781; U.S. Pat. No. 4,654,459; U.S. Pat. No. 4,670,619; U.S. Pat. No. 4,769,508; U.S. Pat. No. 4,777,313; WO 85/00804; and EP 253,522.
Other materials reported as methane coupling catalysts in sequential processes include reducible lanthanide oxides. A. M. Gaffney reports that Pr.sub.6 O.sub.11 (U.S. Pat. No. 4,499,323), CeO.sub.2 (U.S. Pat. No. 4,499,324); and Tb.sub.4 O.sub.7 (U.S. Pat. No. 4,727,212) are effective methane coupling catalysts after doping with alkali metal or alkaline earth metal compounds. The sodium-promoted nonstoichiometric oxide, 4% Na on Pr.sub.6 O.sub.11, is reported to be the most active and selective, giving in the sequential mode 21% methane conversion and 76% selectivity to C.sub.2 and higher hydrocarbons at 800.degree. C. and 1.4 WHSV (weight hourly space velocity) (See Gaffney, et al., J. Catal., 114, 422 (1988)).
A typical response from sequential mode catalysts is that the selectivity to C.sub.2 and higher hydrocarbons increases as the methane coupling catalyst is reduced (methane conversion decreases). Hence, initially when the methane conversion is highest, the selectivity to C.sub.2 and higher hydrocarbons is at its minimum. Thus, catalysts are sought in which the selectivity to C.sub.2 and higher hydrocarbons is high during the early stages of the catalyst reduction when the rate of conversion is the greatest.
M. Vallet-Regi, et al., (J. Chem. Soc. Dalton Trans., 775 (1988)) have disclosed a double perovskite of the formula LaCaMnCoO.sub.6. This material is said to undergo reduction according to the reaction: EQU LaCaMn.sup.4+ Co.sup.3+ O.sub.6 +H.sub.2 .fwdarw.LaCaMn.sup.3+ Co.sup.2+ O.sub.5 +H.sub.2 O.